Previous approaches for sorting orbital angular momentum (OAM) used a cascading series of Mach-Zehnder interferometers as disclosed by Leach, J., Padgett, M. J., Franke-Arnold, S., & Courtial, J. (2002), “Measring the Orbital Angular Momentum of a Single Photon”, Phys. Rev. Lett., 88, 257901, and also disclosed in United States Patent Application No. 2005/0259914. This implementation was large, expensive, and unstable due to the required alignment precision. Recently a highly efficient OAM sorting method was developed utilizing an optical geometric transform which converts helical phase into a linear phase gradient as disclosed by Berkhout, G. C., Lavery, M. P., Courtial, J., Beijersbergen, M. W., & Padgett, M. J. (2010), “Efficient sorting of orbital angular momentum states of light”, Phys. Rev. Lett., 105, 153601. The method has been demonstrated with both diffractive elements as disclosed by Berkhout et al. and refractive elements as disclosed by M. P. J. Lavery, D. J. (2012), “Refractive element for the measurement of the orbital angular momentum of a signle photon”, Optics Express, vol. 20, no. 3, 2110-2115. This scheme solved the size and stability issues as two complex elements in an easily aligned 4-f imaging system can in principle sort an infinite number of OAM states; however, these elements must be individually diamond machined with a 3 axis ultra-precision lathe.
An OAM sorting element that can be quickly and inexpensively replicated would be advantageous.